Lithography, photolithography, and microlithography are techniques used to reproduce patterns. As the requirements of lithography have become more demanding the use of optics has increasingly been applied to lithography. This is especially true in semiconductor manufacturing where very small patterns or circuit geometries are reproduced. The use of optics in lithography to reproduce very small geometries is generally limited to the ability of the optical design to form an image without aberrations. Typically, the larger the image field the more difficult it is to correct for the optically introduced aberrations.
In most lithography applications the image field of the system must be as large as the object being reproduced. Therefore, for large objects having small geometries or patterns thereon the image field of the optical system must be large. However, due to the large image field and small geometries or patterns thereon it is difficult to correct for all of the aberrations introduced by the optical system to obtain a satisfactory image.
Several techniques have been developed in attempting to keep the image field size small, thereby reducing the number of aberrations resulting in better image quality. Two such techniques are step-and-repeat and scanning. In step-and-repeat systems a small image is reproduced multiple times on a larger field in a matrix or array type-fashion. The step-and-repeat technique is most appropriate when the pattern required is repetitive or consists of multiple duplicates of the same object. Scanning involves the use of a good-quality imaging field which typically is relatively large in one dimension and much smaller in an orthogonal direction (e.g., a rectangular or arcuate slit). The longer dimension must be at least equal to one dimension of the object to be reduced. The smaller image field dimension typically is much less than the corresponding dimension of the object to be reproduced. Full coverage of the object is obtained by scanning the field synchronously across the object and the image in a direction nominally parallel to the shorter dimension of the image field. This technique provides better image quality than would be possible with an optical system having an image field as large as the object to be reproduced. In most applications the scanning technique requires an image field at least as large in its longer dimension as the smallest overall dimension of the object being reproduced. For example, a rectangular object of finite width and infinite length can be reproduced if the larger dimension of the image field is as large as the width of the object.
Two patents that disclose optical systems that can be used in lithography and are particularly suited to the scanning technique are U.S. Pat. No. 3,821,763 issued June 28, 1974 to Scott entitled "Annular Field Optical Imaging System", and U.S. Pat. No. 3,748,015 issued July 24,1973 to Offner entitled "Unit Power Imaging Catoptric Anastigmat", which are both incorporated herein by reference. These two patents disclose "ring-field" imaging systems that are advantageous for the reasons stated therein.
One application for these optical lithographic systems is in the volume production of optical disks, magnetic disks, and other disks having patterns thereon in an annular area. These patterns in an annular area may simply be comprised of concentric rings or concentric rings having information encoded thereon. As the information desired to be encoded on these disks becomes greater the number of concentric rings making up these disks necessarily becomes greater. This results in smaller images pushing the limits of current optical lithographical techniques in reproducing them. Existing reproduction techniques mandate that the image field be at least as large as the diameter of the disk being reproduced. Reproducing the ever decreasing geometries of the concentric rings on the disks over such a large image field has become increasingly difficult and costly Therefore, there is a tremendous need to more efficiently reproduce large quantities of disks that simply cannot be done with existing techniques.